1. Field of the Invention
The invention pertains to the field of electrical equipment manufacture and, particularly, the electrical attachment of integrated circuit chips, such as microprocessors and application-specific integrated circuits (ASICS), to printed circuit boards. More specifically, the method of attachment uses a bolster plate to reinforce the printed circuit board during the attachment process.
2. Discussion of the Related Art
A variety of methods have been devised for the attachment of integrated circuit chips, such as microprocessors and ASCIS, to printed circuit boards in a manner that assures consistency in establishing good electrical contact. Many of these methods involve the use of high compressive loads, which are applied to the processors. The integrated circuit chips typically have a plurality of pins that mate with a corresponding female conductive receptacle in the printed circuit board. The printed circuit boards, alone, lack sufficient rigidity to support the compressive loads during attachment. For example, these loads may range from two hundred to three hundred pounds force (890 to 1300 Newtons). Resultant bending of the printed circuit board is capable of damaging wiring or other materials within the integrated circuit chip. Additionally, the bending moment is capable of disrupting the desired electrical contact.
The problem of printed circuit board bending under these heavy loads is typically resolved by using a bolster plate, which is a piece of metal that attaches to the printed circuit board, e.g., by bolting, riveting, or adhesion. The bolster plate may be constructed in any geometrical shape that provides the requisite support. The bolster plate is usually located on the reverse side of the printed circuit board opposite that side on which the integrated circuit chip resides.
Newer microprocessors and ASICS devices have increased numbers of pins in comparison to older devices. Furthermore, the newer devices operate at much higher speeds than did older devices. The increasing number of pins and higher levels of performance demand closer mechanical tolerances for manufacturing purposes. It has been discovered that the use of a bolster plate according to traditional practices does not sufficiently eliminate the bending moment in the printed circuit boards in light of these new demands. In applications where, for example, a force of 270 pounds (1200 Newtons) is applied to seat a microprocessor, a conventional bolster plate may bow a distance of 0.001 inch (0.0025 cm). Even this small amount of bending is sufficient damage the assembly or to cause failure in the electrical contact.
The bolster bow or bend is at maximum in the center of the bolster plate. Additional rigidity could be imparted by increasing the thickness of the bolster plate, but this requires additional room for the bolster plate. The increased thickness creates other difficulties in the context of fitting additional components on the printed circuit board and in assembling adjacent components in the intended use environment. Attempts have been made to pre-bow or pre-stress the bolster plate to accommodate the stress during the attachment of microprocessors, but the resulting bending moment from pre-stressing the bolster plate was not repeatable.
There remains a problem in preventing bolster plate bending due to the insertion of newer microprocessors and ASICS devices.